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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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COLUMBIA GLACIER                                      271
to July 4, 1910. At the same point the advance from July 4 to September 5, 1910, two months, was 132 feet. At this latest observation, the edge of the push moraine in front of the advancing glacier was only S3 feet from the highest point in the middle of the gray-wacke ledge.
At the time of each of the three visits by the National Geographic Society's expeditions the push moraine at the advancing glacier margin showed about the same conditions. There was a gravel ridge parallel to the ice front and 15 to 25 feet high (Pi. CEX, A). Between its crest and the ice was a depression 6 or 8 feet deep, and the nose of the glacier was visibly plowing under the gravel. The outer slope of the push moraine was as steep as sliding gravel will stand. The advance was taking place without disturbance of the beach gravels beyond the push moraine, in this respect contrasting with the conditions along the eastern edge of the glacier which are described later.
The western edge of the western clump of timber, as shown in an 1899 photograph, was not recognizable in August, 1909, and at the time of our visit in 1909, there seemed to be no trees as near the water's edge as in 1899. Since the forest belt recedes farther from the coast as one proceeds southward from the 1899 glacier front this indicates a considerable advance before August, 1909, truncating the edge of the forest, and extending beyond the 1892 maximum. Within the western clump pf timber the ice was pushing in among the trees in August, 1909, overturning them by the thrust against their roots and killing them by burying their trunks beneath the rock and soil of the push moraine which bordered parts of the ice front. Furthermore, the glacier was faulted in places and long splinters of ice were thrust forward in advance of the base of the ice cliff, coming in contact with the upper trunks of mature trees and tipping them over. These upper ice portions were being pushed ahead of those below by thrust faulting, evidently because the basal ice was in contact with the soil and the tangle of roots of a mature forest, so that the less resisted upper layers of the ice slid over those below. There were small living trees actually under the projecting ice front. One could step from the glacier surface into the branches of a tree twenty-five feet above the ground, or climb a tree and step out upon the surface of the projecting upper layer of ice.
The exact amount of advance here is not definitely known, but the ice was surely farther out than it had been for fifty to one hundred years, judging by the age of the trees now being overturned. The forest was mature, with thickly-set trunks and deep moss, and some of the trees were a foot in diameter and from fifty to one hundred years old. That at the time of our visit in August, 1909, the advance at this point had extended beyond the 1892 maximum is proved by the absence of previously overturned trees in this western forest belt, in contrast with the recognizable inclined trees just to the east where the advance had extended up to but probably not very far beyond the 1892 maximum.
Between August, 1909, and July 4, 1910 (Fig. 32), the ice advanced nearly through this western belt of timber, overriding the trees and pushing up a mass of soil, gravel, and wood in front of it. During this period of ten months the width of the forest from north to south was diminished from about 300 feet to 100 feet in the widest place. Between July 4 and September 5, 1910 (Fig. 33), the destruction of this forest continued and on the latter date there was not a tree left standing upright in the main grove. All were either overridden or lay prone (PI. CIX, B), or were held inclined by the moraine or the ice, except a half-dozen semi-detached spruces at the southern border. One of